Of course you sometimes need quiet time alone to work on a programming task. The point is that you'd better also be capable of communicating frequently and effectively with your colleagues. For sure, many introverts are perfectly capable of doing that. (There are also many extroverts who can work alone when the occasion calls for it.) My point is that introversion is not a wholly adventitious trait for a programmer. Looking at the job description objectively, one would not expect programmers to skew strongly towards introversion.